About 7% of 340 adult vampire bats and 33% of 258 younger bats (<2 years of age) failed to feed on a given night. But when they do feed, wild vampire bats fill up like water balloons.

Jon Flanders took this picture of a male vampire bat. He had originally caught this bat around the time of emergence and then caught it again about 3-4 hours later. As you can see, it had just fed and it’s so full it looks pregnant. He described the recapture this way:

Urinating the whole time I was getting it out of the net (I could see where its urine was on the floor underneath it as well). When we weighed it, it was still 6g heavier than when we first caught it.

This is a great illustration of how foraging in vampire bats is a ‘boom-or-bust’ phenomenon, where a successful bat gets either a large meal or none at all.

So the best way to save food for later might be in the form of social capital that leads to future sharing.

Before describing the food-sharing systems of the Ache people of Paraguay and the !Kung San of the Kalahari Dessert, Steven Pinker lucidly explains the basic logic of reciprocal food sharing in How the Mind Works:

…species are driven to share when the variance of success in gathering food is high. Say in some weeks I am lucky and have more food than I can eat, but in other weeks I am unlucky and in danger of starving. How can I store extra food in the fat weeks and draw on it in the lean weeks? … I can store it in the bodies and minds of other people, in the form of a memory of my generosity they feel obliged to repay when fortunes reverse. When the prospects are risky, it pays to pool the risk.

Compared with females, adult male vampire bats do not participate much in reciprocal food sharing. So one would predict that they starve to death more often, and that males may even have slightly different physiological strategies for dealing with fasting. But I’m not sure if there’s any evidence to support this conjecture.

5 Responses to Foraging vampire bats can expect big meals or none at all

How long do female bats nurse their young? I wonder if male bats might be more able to adjust (reduce) their energy expenditure than female bats who have offspring that they need to support. Or maybe I should say that it could be more necessary for female bats to have a constant food supply. So it’s possible that male bats don’t starve to death more even if they don’t share food. Do they participate in parental care?

Yes, you might be right. Males don’t care for young. Females invest a lot. The gestation period can last up to 8 months, the longest known for bats. Maternal care, including nursing and large amounts of regurgitated food sharing, can extend until offspring are sexually mature, which can take up to 9 months. So based on that, females are definitely more in need. On the other hand, my guess would be that male vampires invest less in survival and more in reproductive competition, which is why males sometimes fight to the death over territories. Based on what I’ve gathered from field observations, I picture the females going back to roosts to socialize after feeding, whereas the males are constantly trying to invade other roosts, a different use of energy. This seems typical. For example, in the UK, before hibernation, bats go through something called “swarming”– where males appear to expend most of their energy on traveling and mating, while females appear to save their fat reserves for producing viable offspring the following year. It’s called “swarming” because hundreds or even thousands of bats chase each other and make social calls while flying in and around hibernation sites. But if you set up a net, you only catch males. They are the ones flying around. The females are just chilling out. These transient males come from very distant locations (e.g. 30-60 km away) and sometimes visit each swarming site for only a few hours. Unlike temperate bats, vampire bats are reproductive all the time, and I imagine this happening on a smaller scale continuously.

About

We aim to understand the evolution and regulation of cooperative relationships. I use grooming and food sharing to gain insights into the social lives of vampire bats. My lab starts Fall 2018 at The Ohio State University.